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SubjectRE: [PATCH] kvm: Enable device LTR/OBFF capibility before doing guest device assignment
Date
> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@redhat.com]
> Sent: Tuesday, May 08, 2012 11:18 PM
> To: Hao, Xudong
> Cc: Avi Kivity; Xudong Hao; mtosatti@redhat.com; kvm@vger.kernel.org;
> linux-kernel@vger.kernel.org; Zhang, Xiantao
> Subject: RE: [PATCH] kvm: Enable device LTR/OBFF capibility before doing guest
> device assignment
>
> On Tue, 2012-05-08 at 09:16 +0000, Hao, Xudong wrote:
> > > -----Original Message-----
> > > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > > Sent: Tuesday, May 08, 2012 12:16 AM
> > > To: Hao, Xudong
> > > Cc: Avi Kivity; Xudong Hao; mtosatti@redhat.com; kvm@vger.kernel.org;
> > > linux-kernel@vger.kernel.org; Zhang, Xiantao
> > > Subject: RE: [PATCH] kvm: Enable device LTR/OBFF capibility before doing
> guest
> > > device assignment
> > >
> > > On Mon, 2012-05-07 at 07:58 +0000, Hao, Xudong wrote:
> > > > > -----Original Message-----
> > > > > From: Avi Kivity [mailto:avi@redhat.com]
> > > > > Sent: Sunday, May 06, 2012 11:34 PM
> > > > > To: Xudong Hao
> > > > > Cc: mtosatti@redhat.com; kvm@vger.kernel.org;
> > > linux-kernel@vger.kernel.org;
> > > > > Zhang, Xiantao; Hao, Xudong; Alex Williamson
> > > > > Subject: Re: [PATCH] kvm: Enable device LTR/OBFF capibility before doing
> > > guest
> > > > > device assignment
> > > > >
> > > > > On 05/06/2012 06:24 PM, Xudong Hao wrote:
> > > > > > Enable device LTR/OBFF capibility before do device assignment, so that
> > > guest
> > > > > can benefit from them.
> > > > >
> > > > > cc += Alex
> > > > >
> > > > > > @@ -166,6 +166,10 @@ int kvm_assign_device(struct kvm *kvm,
> > > > > > if (pdev == NULL)
> > > > > > return -ENODEV;
> > > > > >
> > > > > > + /* Enable some device capibility before do device assignment,
> > > > > > + * so that guest can benefit from them.
> > > > > > + */
> > > > > > + kvm_iommu_enable_dev_caps(pdev);
> > > > > > r = iommu_attach_device(domain, &pdev->dev);
> > > > >
> > > > > Suppose we fail here. Do we need to disable_dev_caps()?
> > > > >
> > >
> > > If kvm_assign_device() fails we'll try to restore the state we saved in
> > > kvm_vm_ioctl_assign_device(), so ltr/obff should be brought back to
> > > initial state.
> > >
> > Right, more clear.
> >
> > > > I don't think so. When a device will be assigned to guest, it's be
> > > > owned by a pci-stub driver, attach_device fail here do not affect
> > > > everything. If host want to use it, host device driver has its own
> > > > enable/disable dev_caps.
> > >
> > > Why is device assignment unique here? If there's a default value that's
> > > known to be safe, why doesn't pci_enable_device set it for everyone?
> > > Host drivers can fine tune the value later if they want.
> > >

If host did not have this device driver or host did not load the driver, who will enable them? Guest? But in guest, it really need qemu PCIe support.

> > > > > > if (r) {
> > > > > > printk(KERN_ERR "assign device %x:%x:%x.%x failed",
> > > > > > @@ -228,6 +232,7 @@ int kvm_deassign_device(struct kvm *kvm,
> > > > > > PCI_SLOT(assigned_dev->host_devfn),
> > > > > > PCI_FUNC(assigned_dev->host_devfn));
> > > > > >
> > > > > > + kvm_iommu_disable_dev_caps(pdev);
> > > > > > return 0;
> > > > > > }
> > > > > >
> > > > > > @@ -351,3 +356,30 @@ int kvm_iommu_unmap_guest(struct kvm
> *kvm)
> > > > > > iommu_domain_free(domain);
> > > > > > return 0;
> > > > > > }
> > > > > > +
> > > > > > +static void kvm_iommu_enable_dev_caps(struct pci_dev *pdev)
> > > > > > +{
> > > > > > + /* set default value */
> > > > > > + unsigned long type = PCI_EXP_OBFF_SIGNAL_ALWAYS;
> > > > > > + int snoop_lat_ns = 1024, nosnoop_lat_ns = 1024;
> > > > >
> > > > > Where does this magic number come from?
> > > > >
> > > > The number is the max value that register support, set it as default
> > > > here, we did not have any device here, and we do not know what's the
> > > > proper value, so it set a default value firstly.
> > >
> > > The register is composed of latency scale and latency value fields.
> > > 1024 is simply the largest value the latency value can hold (+1). The
> > > scale field allows latencies up to 34,326,183,936ns to be specified, so
> > > please explain how 1024 is a universal default.
> > >
> >
> > Since each platform will have its own max supported latency, I think
> > the best way is setting the value to 0 because we have such a device
> > now.
>
> What's the benefit to that device vs the risk to other devices?

Default value 0 does not affect any device, right?

> Again,
> if there's a safe default value for both LTR and OBFF, why isn't PCI
> core setting it for everyone? I'm inclined to wait for qemu express
> support and expose LTR/OBFF control to the guest if and only if we can
> enable it on the root complex and intermediate switches. Thanks,
>

Alex, do you means you're working on the qemu express support and ltr/obff exposing? If so, when will this support finish?

Thanks
Xudong


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